Start X11 and web frontends for xC

For this, we needed a wire protocol.  After surveying available options,
it was decided to implement an XDR-like protocol code generator
in portable AWK.  It now has two backends, per each of:

 - xF, the X11 frontend, is in C, and is meant to be the primary
   user interface in the future.

 - xP, the web frontend, relies on a protocol proxy written in Go,
   and is meant for use on-the-go (no pun intended).

They are very much work-in-progress proofs of concept right now,
and the relay protocol is certain to change.

common.c

@@ -1,7 +1,7 @@
 /*
  * common.c: common functionality
  *
- * Copyright (c) 2014 - 2020, Přemysl Eric Janouch <p@janouch.name>
+ * Copyright (c) 2014 - 2022, Přemysl Eric Janouch <p@janouch.name>
  *
  * Permission to use, copy, modify, and/or distribute this software for any
  * purpose with or without fee is hereby granted.
@@ -48,6 +48,66 @@ init_openssl (void)
 #endif
 }
 
+static char *
+gai_reconstruct_address (struct addrinfo *ai)
+{
+	char host[NI_MAXHOST] = {}, port[NI_MAXSERV] = {};
+	int err = getnameinfo (ai->ai_addr, ai->ai_addrlen,
+		host, sizeof host, port, sizeof port,
+		NI_NUMERICHOST | NI_NUMERICSERV);
+	if (err)
+	{
+		print_debug ("%s: %s", "getnameinfo", gai_strerror (err));
+		return xstrdup ("?");
+	}
+	return format_host_port_pair (host, port);
+}
+
+static bool
+accept_error_is_transient (int err)
+{
+	// OS kernels may return a wide range of unforeseeable errors.
+	// Assuming that they're either transient or caused by
+	// a connection that we've just extracted from the queue.
+	switch (err)
+	{
+	case EBADF:
+	case EINVAL:
+	case ENOTSOCK:
+	case EOPNOTSUPP:
+		return false;
+	default:
+		return true;
+	}
+}
+
+/// Destructively tokenize an address into a host part, and a port part.
+/// The port is only overwritten if that part is found, allowing for defaults.
+static const char *
+tokenize_host_port (char *address, const char **port)
+{
+	// Unwrap IPv6 addresses in format_host_port_pair() format.
+	char *rbracket = strchr (address, ']');
+	if (*address == '[' && rbracket)
+	{
+		if (rbracket[1] == ':')
+		{
+			*port = rbracket + 2;
+			return *rbracket = 0, address + 1;
+		}
+		if (!rbracket[1])
+			return *rbracket = 0, address + 1;
+	}
+
+	char *colon = strchr (address, ':');
+	if (colon)
+	{
+		*port = colon + 1;
+		return *colon = 0, address;
+	}
+	return address;
+}
+
 // --- To be moved to liberty --------------------------------------------------
 
 // FIXME: in xssl_get_error() we rely on error reasons never being NULL (i.e.,
@@ -74,6 +134,15 @@ xerr_describe_error (void)
 	return reason;
 }
 
+static struct str
+str_from_cstr (const char *cstr)
+{
+	struct str self;
+	self.alloc = (self.len = strlen (cstr)) + 1;
+	self.str = memcpy (xmalloc (self.alloc), cstr, self.alloc);
+	return self;
+}
+
 static ssize_t
 strv_find (const struct strv *v, const char *s)
 {